1. Field of the Invention
The invention relates to a detector for recognizing the living character of a finger that is arranged in a fingerprint recognizing apparatus. The detector is in contact with a print area of the living finger that constitutes a print forming element, and for the examination of the print area the apparatus comprises a print detector that has a print imaging surface partially covered by the print area, and the detector comprises an electrode system made of an electrically conductive material that senses the presence of the print forming element. The detector further comprises an electrical evaluation unit coupled through electrical contacts to the electrode system, the unit senses the change in state in the electrode system caused by the proximity of the print forming element.
2. Background of the Related Art
The identification of individuals on the basis of fingerprint recognition has become recently a widely used technique. In case of conventional fingerprint analysis using a painted paper as well as in case of opto-electric fingerprint recognition systems the fingerprints are obtained when the tip of a finger is pressed against a surface. The modern opto-electric fingerprint recognition can take place under human supervision (e.g., when the fingerprint is entered in the criminal record) or without any supervision (e.g., in case of access control systems).
In case of fingerprint recognition without human supervision the fingerprint reading apparatus can be deceived by using a plastic imprint copy made from the finger of the person to be identified, thus a false access cannot be excluded. Therefore, in protection or security systems as well as in systems permitting access to computer systems and in any other similar application, in addition to the fingerprint recognition, determining whether the print has been taken from a living finger or from a copy is of vital importance. Moreover, the process of detecting the living character of the finger in a fast and reliable manner is also an important objective.
There are known methods for detecting the living character of a finger. In the EP 0194783 A2 the optical spectral reflection coefficient of the finger is measured at two wavelengths. The measurement of the reflection coefficient takes place at the two free sides of the finger pressed to the fingerprint recognizing apparatus. The quotient of the two reflection coefficients varies during the placement of the finger to the print area from a position where the tip of the finger just touches the surface till the fully pressed state. The detection of the living character of the finger is based on the detected changes of the quotient of the reflection coefficient. The drawback of this technique lies in that if a thin, transparent copy made by a thin film is placed on a living finger of another person, the change of the quotient of the reflection coefficient will be characteristic to a living finger, and a false detection is obtained. Such a system is used in the personal identification apparatus manufactured by the company Biometrics Technology as reported in "The Biometrics Report, ISBN 190018009, 1995. page 69".
In the Japanese patent publication 053309082 an example can be found wherein the measurements are based on the electrical resistance as one property of the print forming element. In this technique an electrode is provided that contacts the surface of the finger just below the first joint, i.e., which is outside the area from where the fingerprint is taken. The electrode is used to detect the electrical resistance of the living finger. The resistance value depends on the humidity of the finger, and if this resistance falls in a predetermined range, the living character will be established. This identification is not reliable either, since the detection takes place at a location which differs from the location from where the fingerprint is taken. If a thin copy is placed between the finger tip and a print area, the contacts can still engage the living finger.
FIG. 34 of the EP 0 359 554 A2 shows an electrical system which also utilizes the difference in the resistance value of a living finger and of a replica, wherein the area used for determining the resistance value falls in the field from where the fingerprint is taken. The cited publication describes this technical solution as one where the electrode patterns used for determining the resistance value may disturb the image of the fingerprint. Furthermore, it has been analyzed that in such a system the allowable resistance value must be very large, however, then the difference between the resistance value of the finger and the replica will become smaller thus the security of such a system can be insufficient.
It can be understood from the above described known methods that they are inappropriate for reliably detecting whether the living finger belongs to the authorized person or not.